Motor vehicles such as dump trucks, tractor trailer trucks, refuse trucks, transit concrete mixing trucks, open-bed trucks, utility trucks, military trucks, and other vehicles of various types to which a significant load may be added for transport are limited in their load transporting ability by various factors. Such as the weight bearing capacity of their supporting axles and applicable federal and state laws. With such laws for example limiting the gross vehicle weight to 80,000 pounds, the weight carried by a single axle to 20,000 pounds and there being an exception as to consecutive axles that limits them to carrying a prescribed combined weight depending on their number and spacing. For example, the combined axle carrying weight is limited to 34,000 pounds in regard to the two powered tandem axles that are typically employed with heavy duty load-transporting motor vehicles. And with such factors as a result impacting the use of such motor vehicles in that the more weight the vehicle can transport at a time, the more useful the vehicle can be provided other factors that impact the ability of the vehicle to perform in an acceptable manner are also taken into account. With such factors including the axle manufacturers rated load capacity.
And in regard to such vehicles wherein it is desired to increase their load carrying capacity, it is common practice to add one or more auxiliary axles to the motor vehicle that are deployed on command to help support the motor vehicle and thereby reduce the weight carried by the axles that normally support the vehicle. Wherein the axles that normally support the vehicle are referred to as primary axles and include at least one axle with steerable wheels and one or more powered axles.
Whereas the auxiliary axles are referred to as either a pusher axle, tag axle or trailing axle in distinguishing between them in how they are employed and help to support the vehicle. Wherein a pusher axle is permanently suspended from the motor vehicle chassis in a location to operate between a forwardly-located primary axle with steerable wheels and one or more rearwardly-located primary axles with powered wheels. A tag axle is permanently suspended from the motor vehicle chassis to operate rearward of one or more primary axles with powered wheels but not normally beyond the rear end of the frame. While a trailing axle (that has also been referred to as a tag axle and trailing tag axle) is also permanently suspended from the motor vehicle chassis but in a manner to operate at a substantial distance rearward the chassis.
On the other hand, the chassis of a trailer when hitched to a motor vehicle chassis does not normally contribute with its one or more axles in helping to support the motor vehicle chassis. And instead, the trailer chassis with or without an added load may depend on the motor vehicle chassis for support in maintaining an upright stance.